It is known that aromatic polyaryl esters or polyarylates may be prepared by transesterification and polycondensation of mixtures of diaryl esters of aromatic carboxylic acids, also together with diaryl carbonates, if desired, and dihydric phenols, and that polycarbonates may be prepared by tranesterification of a diaryl carbonate with a dihydric phenol. It this application, polyarylates are considered to be polycarbonates in which all or part of the carbonic acid residues are replaced by aryl dicarboxylic acid residues, preferably isophthalic and/or terephthalic acid residues.
A disadvantage of known transesterification processes is the tendency of undesirable color or discoloration to form in the resulting polymers. Thus in U.S. Pat. No. 3,299,172, granted Jan. 17, 1967, it is stated (Col. 1, lines 39-43): "Any excess of hydroxyl compounds causes a brown to black discoloration of the polycondensate, because of the insufficient thermal stability of the partial esters of the dihydric phenols still containing free hydroxyl groups which are being formed." The examples in the patent refer to polyesters having yellowish colors.
This problem is also referred to in U.S. Pat. No. 3,395,119 to Blaschke et al at column 2, lines 28-31 where it is stated: "Moreover, excess bisphenol A causes strong discoloration of the polyester because of the thermal instability thereof at the high condensation temperatures employed." The examples in the Blaschke et al patent refer to brownish and yellowish colors of the polyesters which are obtained.